In wavelength division multiplexed (WDM) systems, tunable optical filter systems are typically used in monitoring applications. The filters, however, are also applicable to wavelength add/drop or routing devices, tunable receivers, and tunable sources, for example. Moreover, tunable optical filter systems have relevance to other applications. Remote trace chemical specie detection is one example.
In these applications, wavelength accuracy/stability is desirable. Many times discrimination of a few Gigahertz is necessary, with hundreds of Megahertz being desirable.
Achieving this level of wavelength stability through the design of the tunable filter can be challenging. It requires that the optical cavity length, in the case of a Fabry-Perot tunable filter, be controlled to picometers, for the infrared wavelengths, over the device""s lifetime, which can be many years. Changes in optical cavity spacing, electrostatic cavity spacing (if used), and membrane spring constant can yield measurement shifts. Capacitive sensors typically do not have the stability required to compensate for these shifts.
As a result, many systems utilize reference sources that generate an optical reference signal with stable, known spectral features. The system""s tunable filter is scanned across these spectral features, and the system uses the detected position of the features in a calibration for a subsequent scan of the signal.
The common approach for calibration using a reference source is to periodically switch in the reference source onto the optical fiber pigtail that transmits optical signals to the fiber-based or microelectromechanical system (MEMS) based filter, for example. This allows the system to calibrate the tunable optical filter against the spectral features of the reference source, and then switch to the optical signal for the scan of interest.
There are a number of problems or drawbacks associated with this scheme. First, the process of switching between the optical signal and the reference source can disturb the behavior of the tunable optical filter. This injects some uncertainty into the calibration. Moreover, the resulting system has a relatively low level of integration since the reference source system is not integrated with the tunable filter. Separate fiber pigtails are required between the system and each of the optical signal source and the reference source system.
An alternative approach is to integrate the reference source system with the tunable optical filter on a common optical bench using micro-optical bench technology, for example. This allows the reference source system, tunable optical filter, and possibly a detector system to be integrated together within a single, small hermetic package.
Experimentation, however, has demonstrated that a different set of problems can arise in these relatively highly integrated systems, especially when free space optical interconnects are used. Stray light can exist within the package that can be detected during the signal scan. This has the effect of raising the noise floor of the system, impacting system performance.
The present invention is directed to a tunable optical filter system. It has a reference source that is integrated with the tunable filter. According to the invention, this reference source is temporally modulated to decrease interference or crosstalk into the scan of the optical signal of interest.
In general, according to one aspect, the invention features a tunable optical filter system. The system comprises a package and a tunable filter, which is installed within the package. A reference source system is further commonly installed within the package. This reference source system generates an optical reference that is filtered by the tunable filter and typically used in its calibration. A detector system is further provided that detects the optical reference and an optical signal after being filtered by the tunable filter.
According to the invention, a system controller energizes the reference source during a reference scan in which the tunable filter is scanned across a spectrum of the optical reference. The controller, however, lowers, such as simply decreasing or entirely cutting, power to the reference source system during a signal scan, in which the tunable filter is scanned across the optical signal. In this way, interference during the signal scan from the reference source system is reduced.
In the present implementation, the package is a hermetic package. A butterfly configuration is shown. The tunable filter is a MEMS Fabry-Perot configuration, in the current implementation. The tunable filter can have a single resonant cavity or multiple resonant cavities depending on the required performance.
According to the implementation, the optical reference, which is generated by the optical reference system, comprises stable or known spectral features. The calibration of the tunable filter is made against these spectral features. In one example, the stable spectral features are generated using a light source, e.g., broadband, and a filter that generates the optical reference from the emission of the light source. The combination of a super luminescent light emitting diode (SLED) and fixed Fabry-Perot etalon has been tested.
Depending on the implementation, the detector system can comprise single or multiple detectors. In a multidetector configuration, a first detector can detect the optical reference and a second detector can detect the optical signal. Path separation can be achieved using a dichroic filter. Different orders of the filter are preferably used to scan the optical reference and the optical signal. In an alternative implementation, the detector system comprises a detector that detects both the optical reference and the optical signal.
In one embodiment, the controller ramps the tuning voltage to the tunable filter to perform the reference scan either before or after the signal scan. In the preferred embodiment, however, the reference scan is performed both before and after the signal scan to enable a two point calibration on either side of the optical signal""s spectrum. These tuning voltage ramps can be increasing or decreasing, linear or non-linear, ramps. Finally, in one implementation, the controller entirely removes power to the reference source during the signal scan. Alternatively, however, the controller can simply reduce, but not entirely turn-off, power to the reference source system. This later approach can mitigate shifts due to thermal transients, for example.
In general, according to another aspect, the invention features a method for controlling a tunable optical filter system. This method comprises driving a tunable optical filter to scan over an optical reference spectrum and an optical signal spectrum. A reference source system is controlled to generate the optical reference, while the tunable filter is scanning the optical reference spectrum. While the tunable filter is scanned over the optical signal spectrum, the power to the optical reference system is lowered.
The step of driving the tunable filter can comprise ramping the drive voltage to scan the optical reference spectrum and then ramping the drive voltage to scan the optical signal spectrum. In the scan of the optical reference spectrum, the optical signal can be blocked from reaching the tunable filter.
In a preferred embodiment, the step of driving the tunable filter comprises scanning the optical reference spectrum before and after the scan of the optical signal spectrum.